FIG. 8 is a schematic drawing illustrating a radiation detector that uses a scintillation fiber. In FIG. 8, 1 denotes radial rays. 2a and 2b denote light pulses derived from fluorescence. 3 denotes a cable including a scintillation fiber. 4a and 4b denote light receiving elements coupled to the scintillation fiber 3. 5a and 5b denote head amplifiers. 6a and 6b denote constant fraction discriminators (pulse height discriminators). 7 denotes a signal delay circuit. 8 denotes a time-to-pulse height converter. 9 denotes an analog-to-digital converter. 10 denotes a multichannel pulse height analyzer (radiation analyzer). Operations thereof will be described below. When the radial rays 1 enter into the scintillation fiber 3, fluorescence is generated within the scintillation fiber 3. As a result thereof, the light pulses 2a and 2b propagate to the respective ends of the scintillation fiber 3. The light receiving elements 4a and 4b convert the received light pulses 2a and 2b to electric pulse signals, respectively. The electric pulse signals are input into the time-to-pulse height converter 8 via the constant fraction discriminators 6a and 6b and the signal delay circuit 7. An electrical pulse having a pulse height in proportion to a time lag between arrival timing to the light receiving element 4a and arrival timing to the light receiving element 4b is output from the time-to-pulse height converter 8. The output pulse is input into the analog-to-digital converter 9. The multichannel pulse height analyzer 10 discriminates the pulses for each pulse height to count them. This makes out a position of incidence of the radial rays 1. The count number makes out a radiation dose rate. In the above description, coupling of the light receiving elements 4a and 4b to the respective ends of the scintillation fiber 3 makes out the position of incidence of the radial rays 1. This is referred to as the Time of Flight (TOF) method (method for measuring time lag in flight). This is a typical radiation dose measuring method listed in the following citation list. Only one light receiving element is sufficient for measuring the only radiation dose rate.
A plastic scintillation fiber (PSF) is known as the scintillation fiber 3. The PSF is a plastic fiber including a plastic scintillator (core) clad in a low refractive index polymer. The core is typically configured such that an organic fluorescent substance is dissolved in an organic polymer (e.g., polystyrene and polyvinyl toluene) having an aromatic ring. The low refractive index polymer is, for example, polymethyl methacrylate or fluorine-containing polymethyl methacrylate. The PSF is employable for the use of the radiation measurement. A principle of the radiation measurement using the PSF is as follows. When the radial rays (e.g., high energy electromagnetic waves such as X-rays and γ-rays; neutron rays; electron rays (β-rays); charged particle radiation such as protons) are radiated to the core, ultraviolet rays are emitted from the high polymer of the aromatic ring as a core material. As a result thereof, the contained organic fluorescent substance induces absorption of ultraviolet radiation and wavelength conversion to a long wavelength. More specifically, the contained organic fluorescent substance induces conversion to a blue color that represents the maximum sensitivity of a photomultiplier tube. The blue color light propagates through the core to reach each of the light receiving elements where it is detected.
FIG. 9 is a schematic drawing illustrating a luminescence principle by irradiation of the radial rays to the PSF. When the radial rays cross the PSF, the poly styrene (PS) constituting the core emits ultraviolet radiation having a wavelength around 300 nm. The core made of the PS contains, for example, two different kinds of fluorescent agents (e.g., a primary fluorescent agent and a secondary fluorescent agent) (see, FIG. 10). The ultraviolet radiation is converted into light having a wavelength of around 350 nm by the primary fluorescent agent, and the light having the wavelength around 350 nm is further converted into visible blue color light having a wavelength around 430 nm by the secondary fluorescent agent. At a position within a range of several mm from the position at which the radial rays entered, the wavelength conversion from the ultraviolet radiation having the wavelength of around 350 nm to the visible blue color light having the wavelength of around 430 nm completes (see, FIG. 9). The visible blue color light partially propagates in a longitudinal direction of the PSF to the respective sides of the PSF. The light receiving elements (e.g., photomultiplier tube and PMT) that received the visible blue color light output electric signal pulses. Intensities of the visible blue color light reaching the ends of the PSF are normally weak, i.e., are a level within a range between several photons and several tens of photons (light quantum). FIG. 11 illustrates an example of the radiation detection assembly having a simplest structure using the PSF. When the radial rays 1 cross the PSF cable 3, many photons having a blue color wavelength are generated through the core of the PSF. The light pulse 2 of a portion thereof propagates to the right (rightward in FIG. 11) within the PSF. The blue light of the range between several photons and several tens of photons reaches a photomultiplier tube coupled to a right end face of the PSF as the light pulse 2 having a time width of several nanoseconds. The radial rays cross the PSF cable 3 at random times. Therefore, the light receiving elements (e.g., photomultiplier tubes) 4 output electric pulse signals at random times. The electric pulse signals are amplified by the head amplifiers 5. The constant fraction discriminators (pulse height discriminators) 6 convert only signals of a level higher than the noise level into rectangular pulses. A multichannel scaler 11 counts the number of the rectangular pulse signals per unit time to measure the radiation intensity thereof. In a lower part of FIG. 11, changes of signals are schematically illustrated with the horizontal axis being time and the vertical axis being signal strength. The changes of signals are illustrated in the order that the radial rays are entered into the PSF, the radial rays are converted into the optical pulse signals, thus obtained optical pulse signals are further converted into the electric pulse signals, and the resulting signals are finally counted.
In a case where the photomultiplier tube is employed as the light receiving element, it is desirable to use the blue color light-emitting scintillation fiber. In a case where silicon photo diode is used as the light receiving element, since the silicon photo diode is more sensitive to a longer wavelength, it is desirable to use a green color light-emitting scintillation fiber having a wavelength of around 500 nm, more desirably, a red color light-emitting scintillation fiber having a wavelength of around 600 nm. To achieve this, various kinds of scintillation fibers made by appropriately selecting kinds and combinations of phosphors to be dissolved in the polystyrene are used.
To enhance measurement sensitivity, it is possible to use the PSF having a diameter of a range between 2 and 5 mm.
In a case where the PSF having a length beyond three meters up to several tens meters is used, it is essential for the PSF cable to have flexibility for the sake of accommodation of the PSF cable and measurement with the PSF cable being bent. In consideration of the measurement sensitivity and the flexibility of the PSF, a cable having a configuration that a plurality of PSFs each having a diameter less than 2 mm is bundled to be accommodated in the protective tube is proposed. In the above situation, to enhance the flexibility of the cable, it is preferable that the protective tube and the PSF are not tightly coupled to each other, i.e., are not formed into one piece, but are only assembled loosely. Further, it is preferable that the plurality of PSFs is not bound into one body but is restricted loosely. It is preferable that the plurality of PSFs is simply accommodated in the protective tube spaced constantly one another.